PHYSICS CH 16

1. PHYSICS CH 16 Electrical Forces and Fields

2. Static Electricity Events • Clothes tumble in the dryer and cling together. • You walk across the carpeting to exit a room and receive a door knob shock. • Sparks of electricity are seen as you pull a wool blanket off the sheets of your bed. • You stroke your cat's fur and observe the fur standing up on its end. • Bolts of lightning dash across the evening sky during a spring thunderstorm. • You have a bad hair day. • These events can only be explained by an understanding of the physics of electrostatics.

3. Electrostatic Forces • Without the forces associated with static electricity, life would be impossible. • Electrostatic forces hold the world of atoms and molecules together. • All material objects are made of atoms • The acts of standing and walking, touching and feeling, smelling and tasting, and even thinking is the result of electrical phenomenon. • Electrostatic forces are foundational to our existence.

4. Quick Review of Atoms • Atoms • Nucleus • Contains positively charged protons and neutral neutrons. • Protons and neutrons are not removable everyday methods and will remain within the nucleus of the atom • Electrostatic phenomenon can never be explained by the movement of protons or neutrons • Protons (p, +) • Positively charged • Neutrons (n,0) • No charge • Electron Cloud • Vast region of space outside the nucleus. • Electrons (e,-) • Negatively charged • Attracted to the positively charged nucleus • Electrons are often removed from and added to an atom by normal everyday occurrences. • Electrostatic phenomenon can be explained by the movement of electrons

5. Application of Atomic Structure to Static Electricity • All material objects are composed of atoms. • There are different kinds of atoms known as elements • Elements can combine to form compounds. • Different compounds have distinctly different properties. • Material objects are composed of atoms and molecules of these elements and compounds, thus providing different materials with different electrical properties.

6. Electrically Neutral vs. Electrically Charged • The charge of a proton = the charge of an electron 1+ = 1- equal but opposite • Electrically neutral • Atom that contains equal numbers of protons and electrons • Electrically charged • Atom that has an unequal number of protons and electrons • Referred to as an ion • Positively charged • Any particle that contains less electrons than protons • Negatively charged • Any particle that contains more electrons than protons

7. 11p 9p 9p 11p

8. Charge • Measurable quantity. • Units are Coulombs(C) • The charge on a single electron is -1.6 x 10 -19 Coulomb. • The charge on a single proton is +1.6 x 10 -19 Coulomb. • 1 Coulomb of charge is an abnormally large quantity of charge, • An object with an excess of 6.25 x 1018 electrons to have a total charge of -1 C • An object with a shortage of 6.25 x 1018 electrons would have a total charge of +1 C. • We simplify this by using 1- for electrons and 1+ for protons

10. Charge Interactions • Oppositely charged objects will exert an attractive influence upon each other. • Apositively charged object will exert a repulsive force upon a second positively charged object. • Similarly, a negatively charged object will exert a repulsive force upon a second negatively charged object.

11. Transfer of Electric charge Conductors Insulators Materials that impede the free flow of electrons from atom to atom and molecule to molecule. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging. • Materials that permit electrons to flow freely from atom to atom and molecule to molecule. • An object made of a conducting material will permit charge to be transferred across the entire surface of the object until the overall repulsive forces between excess electrons is minimized.

12. Semiconductor • Have electrical properties somewhere between conductors and insulators • Superconductors • Can conduct electricity indefinitely at certain temperatures • No “resistance”

13. Polarization • The process of separating opposite charges within an object. • By inducing the movement of electrons within an object, one side of the object is left with an excess of positive charge and the other side of the object is left with an excess of negative charge.

14. Methods of Charging • Friction • Induction • Conduction • Grounding

15. Charging by Friction • Results from a transfer of electrons between the two objects that are rubbed together. • Rubber has a much greater attraction for electrons than animal fur. • The atoms of rubber pull electrons from the atoms of animal fur. • The rubber balloon has an excess of electrons (negatively charged) and the animal fur has a shortage of electrons (positively charged).

16. Charging by Induction • Method used to charge an object without actually touching the object to any other charged object.

18. Charging by Conduction • Involves making the physical connection of the charged object to the neutral object. • Often called charging by contact.

19. Grounding - the Removal of a Charge • Process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size. • A ground is simply an object that serves as a seemingly infinite reservoir of electrons; the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object.

20. Electric Force • Vector Quantity • Units are Newtons (N) • Magnitude and direction. • Direction of electrical force depends upon whether the charged objects are charged with like charge or opposite charge and upon their spatial orientation.

21. Coulomb's Law Equation • States that the electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects • Equation form, • q1represents the quantity of charge on object 1 (Coulombs), • q2represents the quantity of charge on object 2 (Coulombs) • r represents the distance of separation between the two objects (in meters) • kis Coulomb's law constant. • The value of this constant is dependent upon the medium that the charged objects are immersed in. • In the case of air, the value is approximately 9.0 x 109 N • m2 / C2.

22. Sample Problem 1 • An electron and a proton are separated, on average, by a distance of 5.3 x 10-11 m. Find the magnitude of the electrical force between the two particles.

23. Sample Problem 2 • Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 Newton. Determine the separation distance between the two balloons.

24. The Principle of Superposition • The resultant force on any single charge is equal to the vector sum of all the forces on that charge.

25. Put in a good sample problem.

26. Electric Fields • Field forces • Found in the space around a charged object • Charged objects that enter each other’s electric fields are repelled or attracted to each other.

27. Electric Field Lines • Lines that point in the direction of an electric field • Allow the visualization of an electric field • Do not really exist http://lectureonline.cl.msu.edu/~mmp/kap18/RR447app.htm

28. Electric Field Line Rules • Lines begin on positive charges or infinity • Lines terminate at negative charges or infinity • The number of lines drawn is proportional to the magnitude of the charge • No two field lines from the same field can cross each other.

33. Electric Field Strength Equation Version 1 • The magnitude of the electric field is simply defined as the force per charge on the test charge. • Newton/Coulomb or N/C.

34. Electric Field Strength Equation Version 2

35. Sample Problem • Find the electric field strength on a test charge that is .03m from a 15μC charge.